This application claims the priority of Korean Patent Application No. 2003-61371, filed on Sep. 3, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method of and an apparatus for compensating for nonlinear distortion, and more particularly to, a method of and an apparatus for compensating for nonlinear distortion for dividing audio signals reproduced in a nonlinear speaker system into linear and nonlinear components in a time domain and a frequency domain, and then generating inversely-corrected signals by means of an inverse filtering scheme.
2. Description of the Related Art
A variety of audio/video (AV) devices such as television sets and audio record players generate audio signals as their final outputs. The audio signals are usually generated by a speaker which converts electrical audio signals into sound pressure waves. A speaker system usually comprises voice coils, a magnet unit surrounded by the voice coils, and a diaphragm which produce physical signals propagating through space from the electrical signals. However, the diaphragm installed in the speaker system does not produce its displacement X in linear proportion to the amplitude of an input signal due to its inherent physical properties. This is because the stiffness of the diaphragm is not linearly proportional to the displacement of the diaphragm. Therefore, the sound pressure waves output according to the nonlinearity contain nonlinear components, which will cause degradation of the sound quality of a variety of audio outputs.
FIG. 1 shows a conventional method for reducing nonlinear distortion.
The input signal Ugl is a signal subjected to a Fourier frequency transform, and is input to a displacement filter 101. The displacement filter 101 has the displacement of vibration as a frequency function, whereby the stiffness k2 can be calculated. Such parameter information for the displacement filter 101 is usually available from a table previously provided by the speaker manufacturer. If the stiffness k2 and the corresponding displacement x are determined, the function f(k,x)=k2x3 can be calculated, and the resulting signal and the input signal Ugl are summed in an adder 103 to generate an inversely-corrected signal Ugn which is input as a final signal to the speaker.
According to the conventional method described above, since the speaker system is modeled by using the lumped parameter method, the applicable frequency band is limited to the range of 500 Hz or less in which the wavelength is larger than the size of the speaker, and thus it is impossible to analyze any nonlinear distortion in the range of 500 Hz or more. Considering that second and third harmonic components which are nonlinear components critically degrading sound quality are generated in the range of 500 Hz or more, the lumped parameter method is not appropriate for nonlinear distortion analysis even if the frequency band of the audio signal is 500 Hz or less.
In the conventional method, the mass M, the stiffness k0, and the viscous damping coefficient R are used to represent the speaker system, and nonlinear stiffness and force factors are assumed as those causing nonlinear characteristics to obtain the equation of nonlinear motion. However, there are various other factors that can actually cause nonlinearity of the speaker system, such as nonlinear viscous damping and structural damping. Furthermore, in the conventional method, the hysteresis phenomenon based on a time history cannot be considered.
In addition, in the conventional method, it is necessary to measure the nonlinear distortion caused by the displacement x of the speaker itself. This actually requires special equipment, thereby causing many difficulties in implementation. Furthermore, it is impossible to reflect phase information of the input signal corresponding to its frequency.